1. Field of the Invention
The present invention generally relates to a charge/discharge protection circuit for a secondary battery and, more particularly, to a charge/discharge protection circuit protecting a charge/discharge control field effect transistor (FET) from overheating due to repetition of on and off. The present invention also relates to a battery pack incorporating such a charge/discharge control field effect transistor, and an electronic device using such a battery pack.
2. Description of Related Art
Many portable electronic devices use a lithium-ion secondary battery. The lithium-ion secondary battery has a problem in that an accident may occur when being overcharged. Moreover, the lithium-ion secondary battery has a problem in that a number of charge/discharge cycles within a service life is reduced if the lithium-ion secondary battery is over-discharged.
In order to solve such a problem, a protection switch is provided to a charge/discharge path between a lithium secondary battery and a main part of a device. The protection switch is turned off so as to prevent the lithium-ion secondary battery being further overcharged or over-discharged when a state (overcharged state) where the lithium-ion secondary battery is charged to have a voltage more than a predetermined voltage or a state (over-discharged state) where the lithium-ion secondary battery is discharged to have a voltage below a predetermined voltage is detected.
The charge/discharge protection circuit, which protects a secondary battery by detecting an overcharged state, an over-discharged state or an over-current so as to cut off a charge/discharge path to the secondary battery, is disclosed in, for example, Japanese Laid Open Patent Applications No. 11-103528, No. 10-285810 and No. 9-182283 and Japanese Patent Publication No. 2872365.
In the above-mentioned conventional charge/discharge protection circuit, an overcharge detection circuit has a hysteresis. That is, if an overcharge of a secondary battery is detected, a charge control field effect transistor (FET) is turned off so as to establish a charge unable state, and, thereafter, the charge control FET is turned on so as to establish a charge enable state when a battery voltage returns to a charge return voltage which is lower than an overcharge detection voltage.
However, in a semiconductor device of the conventional protection circuit, if a deteriorated battery having a high internal-impedance caused by repeated charge and discharge is charged by a charger having a normal current value but a large voltage value, or if a battery having a normal internal-impedance is charged by a charger having a large voltage value and a large current value, the battery voltage drops when a charge current path is cut off (the charge control FET is turned off) due to an overcharge state being detected by an overcharge detection circuit. Under such circumstances, the battery-voltage drop may exceed the hysteresis of the over-discharge detection circuit, and the battery voltage may reach the level of the over-discharge return voltage.
In such a case, a charge operation is started again by turning the charge control FET on, and, thereafter, the same operation is repeated. That is, an oscillating operation occurs in that detection of overchargexe2x86x92cut off of a charge current (turn off the charge control FET)xe2x86x92drop of the battery voltagexe2x86x92start of charge (turn on the charge control FET)xe2x86x92detection of overcharge are repeated. In such a case, there is a risk in that the charge control FET generates heat, which causes an ignition in the worst case.
It is a general object of the present invention to provide an improved charge/discharge protection circuit in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a charge/discharge protection circuit which prevents a switching FET from being overheated or ignited so as to improve safety by preventing an oscillating operation of the switching FET due to repeated detection of overcharge and a returning operation in a case where a battery having a high internal impedance is charged by a charger having a normal current value but a large voltage value and also a case where a battery having a normal internal impedance by a charger having both a large voltage value and a large current value.
Another object of the present invention is to provide a battery pack having the above-mentioned charge/discharge protection circuit and an electronic device, such as a cellular phone, which uses such a battery pack.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a charge/discharge protection circuit comprising: an overcharge detection circuit which detects an overcharge of a secondary battery connected to the charge/discharge protection circuit and outputs an overcharge detection signal; a discharge over-current detection circuit which detects a discharge over-current of the secondary battery and outputs a discharge over-current detection signal; a charge control field effect transistor (FET) connected to a charge path of the secondary battery in series so as to cut off a charge current supplied to the secondary battery; a discharge control field effect transistor (FET) connected to the charge path of the secondary battery in series so as to cut off a discharge current supplied from the secondary battery; a latch circuit which latches the overcharge detection signal output from the overcharge detection circuit and outputs a signal so as to control the charge control FET; a first delay circuit which delays the discharge over-current detection signal output from the discharge over-current detection circuit and supplies the delayed discharge over-current detection signal to the discharge control FET; and a reset circuit which resets the latch circuit so as to turn on the charge control FET when the discharge over-current is detected by the discharge over-current detection circuit and when the overcharge is not detected by the overcharge detection circuit.
In the above-mentioned invention, the reset circuit may cause the latch circuit to continuously output the signal to turn on the charge control FET when the over-current detected by the discharge over-current detection circuit disappears during a delay time of the delay circuit. Additionally, the reset circuit may includes: a second delay circuit which delays the charge over-current detection signal for a predetermined time; and a logic circuit which logically operates the overcharge detection signal after being delayed by the second delay circuit and the discharge over-current detection signal output by the discharge over-current detection circuit. Further, the logic circuit may includes: an inverter inverting the overcharge detection signal after being delayed by the second delay circuit; and an AND circuit performing an AND operation on the inverted overcharge detection signal and the discharge over-current detection signal output by the discharge over-current detection circuit.
Additionally, the charge/discharge protection circuit according to the present invention may further comprise a logic circuit between the first delay circuit and the discharge control FET so as to maintain the discharge control FET being turned on when the latch circuit is reset during a delay time of the first delay circuit. The logic circuit may include: a first inverter inverting the discharge over-current detection signal output from the discharge over-current detection circuit; a second inverter inverting the discharge over-current detection signal inverting the discharge over-current detection signal after being delayed by the first delay circuit; and an OR circuit which performs a summing operation on outputs of the first and second inverters and supplies a result of the summing operation to discharge control FET.
Additionally, according to other aspects of the present invention, there are provided a battery pack using the charge/discharge protection circuit and an electronic device having such a battery pack.